The present invention relates to a method of producing an insulation layer, in particular for a heating device of a sensor, for determining an oxygen concentration in gas mixtures, in particular exhaust gasses of internal combustion engines. The present invention also relates to a sensor for determining an oxygen concentration in gas mixtures.
Sensors of the above-mentioned general type are known in the art. Such sensors serve for determination of the oxygen concentration in the exhaust gas of the internal combustion engine and adjustment of a fuel-air mixture for operating the internal combustion engine. The fuel-gas mixture can be provided in a so-called rich region. In other words, the fuel is in stochiometric excess, so that in the exhaust gas only a small quantity of oxygen with respect to other, partially non-burnt components is available. In the so-called poor region, in which the oxygen exceeds the air in the fuel-air mixture, an oxygen concentration in the exhaust gas is correspondingly high.
In order to determine the oxygen concentration in the exhaust gas, so-called lambda probes are known. In the poor region a lambda value is more than 1, while in the rich region it is less than 1, and in the stochiometric region a lambda value is equal to zero. A Nernst measuring cell of the sensor in a known manner produces a detection voltage, which is supplied to a switching device. The detection voltage depends on an oxygen concentration difference at an electrode exposed to the measuring gas and a reference electrode of the Nernst measuring cell exposed to a reference gas. The detection voltage increases or decreases with the oxygen concentration in the exhaust gas. A solid electrolyte body is arranged between the electrodes of the Nernst measuring cell and is conductive for the oxygen ions.
Such sensors must be heated in an active region of temperatures over approximately 300xc2x0 C., for reaching the required ion conductivity of the solid electrolyte. The operational temperature is achieved by an additionally arranged heating device. The heating device has for example a meanderingly arranged heating conductor which is covered by an insulation layer from the solid electrolyte. The heated conductor is composed for example of a platinum conductor track.
The insulation layer is produced in known methods by addition of aluminum oxide and silicon dioxide-containing flux means, by sintering. The flux medium can be for example Celsian (BaAl2Si2O8)-forming flux means initial material mixtures.
The insulation layer must satisfy the following requirements. On the one hand it must guarantee a sufficiently high mechanical stability to withstand loads occurring during the operation, on the other hand the insulation layer must be as homogenous as possible for minimizing locally occurring leakage current and suppressing damaging actions on the mechanic stability of the insulation layer and/or solid electrolyte. The known methods have the disadvantage in the homogenous structure of the insulation layer and a residual porosity which is technically difficult to reproduce.
Due to the required operational temperatures of 300xc2x0 of the sensor, the electrical conductivity of the flux means-containing aluminum oxide increases, which forms the insulation layer. Partially, in hot condition a leakage current can occur, and the oxygen ions further flow into the solid electrolyte. With sufficiently open porosity of the insulation layer, air serves as the oxygen source. In the case that due to the reduced porosity the oxygen supply from the air is presented, the oxygen drains the solid electrolyte, or in other words the zirconium dioxide grate. The partial reduction of the solid electrolyte which can be visible as the occurring black coloring, provides for an electron conductivity which passes the sensor element as an avalanche. The partial reduction is accompanied by a phase conversion of the solid electrolyte. Because of the phase conversion from metastable tetragonal ZrO2 grains into monokline ZrO2 grains with greater volumes, released tension can lead to crack formation and thereby the heater can be mechanically damaged.
The porosity of the insulation layer in the known methods is significantly dependent from the dry grinding of the row mixture, from the distribution of the flux material barium and silicon, from the paste preparation and the screen printing conditions. The adjustment of the parameters is necessary and the reproducibility is limited, so that during the production an increased fraction is produced as a reject.
Furthermore, the occurrence of the above described leakage current leads to a shortened heater service life, and with the compact construction of the sensor can lead to a complete functional breakdown.
Accordingly, it is an object of the present invention to provide an insulation layer which avoids the disadvantages of the prior art.
It has been found that an insulation sheet can be produced with a homogenous porosity and in a reproducible way, when the production is performed with a mixture of only aluminum oxide, barium oxide and/or strontium oxide and/or during sintering by thermal disintegration of raw materials which form such oxides.
The additional barium oxide and/or strontium oxide can be provided in pure form or in the form of a compound. Preferably, the compound can be barium carbonate or strontium carbonate. The weight fraction of the barium or strontium carbonate during production of the insulation base material is between 3% and 20%. preferably of 9%. The other ingredient of the insulation base material is aluminum oxide, preferably xcex3-aluminum oxide.
The above-identified composition of the insulation base material does not contain silicon oxide, in contrast to the known methods. The fraction of barium oxide or strontium oxide-containing component is substantially increased. Thereby, on the one hand, the sintering temperature, which is required for the thermal production, is lowered below 1400xc2x0 C., and, on the other hand, the formed insulation layer has a homogenous porosity. The utilization of glass-forming silicate flux material in the known methods led to an amorphous insulation layer. The glossy rigid phases close the pores of the heater insulation required for the oxygen supply and have increased ion conductivity (cation-conductivity).
The silicate-free insulation base material in accordance with the present invention makes it possible to avoid formation of amorphous structures during the thermal production. The addition of barium oxide and/or strontium oxide-containing compounds led in an unexpected manner to an especially sintered-active phase transition, which is caused by thermal decomposition with formation of special reactive oxides (Hedvall effect). In the same manner the phase transition of xcex3-aluminum oxide grains to alpha aluminum oxide grains increases the sinter activity of the insulation layer. The barium and/or strontium aluminate grains formed during sintering impart a high strength to the insulation layer. Furthermore, by addition of porogens, for example carbonates, the porosity of the insulation layer can be influenced in accordance with the desired objectives. Generally speaking, the use of the inventive insulation base material leads to a homogenous porosity of the insulation layer.
The preparation of the insulation base material is simplified when compared with the known methods, since the distribution of the porosity in the insulation layer substantially depends on the barium or strontium-containing components. The disadvantages which occur due to sinistrum-containing flux means as described herein above, could be avoided in the known methods only with consideration of numerous parameters, such as for example performing of dry milling or paste preparation.
Generally speaking, with the inventive method the service life of the sensor is substantially increased. The reduction of the disturbing leakage current with the resulting blackening of the solid electrolyte and crack formation in the insulation layer leads to a substantial reduction of rejects.
The novel features which are considered as characteristic for the present invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.